J. Phys. Chem. A 2003, 107, 2497-2504
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Direct Kinetic Study of the Reaction of Cl2•- Radical Anions with Ethanol at the Air-Water Interface R. S. Strekowski,† R. Remorov,†,‡ and Ch. George*,† Laboratoire d’Application de la Chimie a` l’EnVironnement (UCBL-CNRS), 43 bouleVard du 11 NoVembre 1918, F-69622 Villeurbanne, France, and Institute of Chemical Physics, Russian Academy of Science, 4 Kosygin Street, Moscow 117977, Russia ReceiVed: May 24, 2002; In Final Form: NoVember 26, 2002
This work focuses on the development and application of the UV diffuse reflectance-laser flash photolysis technique to directly study the kinetics of reactions occurring at the gas-liquid-phase boundary. The reaction of Cl2•- radical anion with ethanol was chosen to directly “probe” the reaction kinetics at the air-water surface. The reaction rates at the surface are shown to be more rapid than in the bulk liquid. Direct kinetic evidence is provided that the reaction of Cl2•- radical anion with ethanol is at least 2 times faster at the surface than in the bulk. The rate coefficient for the surface reaction Cl2•- + ethanol is found to be (4.45 ( 0.80) × 105 M-1 s-1. For comparison, the rate coefficient for the reaction of Cl2•- with ethanol in the aqueous phase is found to be (1.77 ( 0.34) × 105 M-1 s-1. The uncertainties in the above expressions are 2σ and represent precision only. The effective rate coefficient for the aqueous-phase reaction Cl2•- + ethanol is found to be consistent with what has been reported. Therefore, the chemistry at the interface differs from that of the gas phase or liquid phase. The nature of interfacial reactions and their atmospheric implications are discussed.
Introduction Heterogeneous gas-liquid interactions have been shown to play an important role in atmospheric processes such as the destruction of the stratospheric ozone layer, nonozone compliance in urban or rural environments, acid rain, global warming, the production of photochemical smog in urban-rural settings, and the formation of cloud condensation nuclei.1-3 The laws governing the kinetics of heterogeneous reactions are generally very complex. Typically, the gas uptake into a liquid is governed by several processes, including gas-phase diffusion, mass accommodation, solubility, and liquid-phase chemical reaction.4-6 For many soluble and less soluble atmospheric species the uptake of gas by water surfaces is measured to be entirely consistent with calculations based on the bulk aqueous chemistry of the species.4,7,8 However, there has been increased recognition and a growing body of field9 and laboratory data9,10 that suggest that processes at the air-water interface can play a key role in the uptake and reactions of atmospheric gases with liquid droplets. That is, more efficient reactions may occur at the airwater interface itself. Atmospheric species may react at the interface without actually being taken up into the bulk of the solution.5 For instance, field observations at a North American coastline suggest that an unrecognized chlorine source must exist which cannot be described by a known sequence of gas-phase and heterogeneous reactions.9 Also, the uptake of gaseous SO2 into a liquid cannot be explained by liquid-phase chemistry alone.5,10-12 It has been shown that the atmospheric oxidation of SO2(g) proceeds via the formation of a bound complex at * To whom correspondence should be addressed. E-mail:
[email protected]. † Laboratoire d’Application de la Chimie a ` l’Environnement (UCBLCNRS). ‡ Russian Academy of Science.
the air-water interface.5,11,12 Similarly, the atmospheric uptakes of acetaldehyde4,13 and glyoxal14 cannot be described solely by liquid-phase chemistry and are best explained in terms of enhanced reactivity at the gas-liquid interface. In another example, the uptake of Cl2(g) and Br2(g) by NaBr and NaI solutions cannot be explained with a simple aqueous-phase reaction mechanism and is best described if an additional channel at the gas-liquid interface participates in the reaction.6,15,16 Moreover, George and co-workers observed that the uptake of gaseous BrCl17 and ClNO218 on NaI solutions was driven by bulk-phase chemistry but an additional surface reaction channel was found. Finlayson-Pitts and co-workers report that Cl2(g) is produced when deliquesced sea salt particles are irradiated using 254 nm radiation in the presence of O3.10 The obtained experimental results are only explained in terms of ion-enhanced interactions with gases at aqueous interfaces.10 Clearly, the reactions occurring at the air-water interface are of great potential importance in the field of atmospheric sciences. Although the kinetics and dynamics of many of the gas- and liquid-phase reactions are more or less well understood, the reactions at the air-water interface are not. To date, there are no reported studies employing direct measurements of the reaction kinetics at the air-water interface. The goal of this research is aimed to better understand and evaluate the role of interface processes. This work focuses on the development and application of the diffuse reflectance-laser flash photolysis (DR-LFP) technique to study the reactions occurring near the gas-liquid surface. The reaction of Cl2•- radical anions with ethanol was chosen to directly probe the reaction kinetics close to the air-water interface, because of the appropriate optical properties of that radical. Accordingly, in this paper we report a diffuse reflectance study of the air-water interface (or close to it) kinetics of the
10.1021/jp026174f CCC: $25.00 © 2003 American Chemical Society Published on Web 03/20/2003
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Figure 1. Schematic diagram of the DF-LFP apparatus employed to study the interface kinetics of the Cl2•- + ethanol reaction. HCA ) highcurrent amplifier, PMT ) photomultiplier tube, DG535 ) digital delay, and PC ) personal computer.
reaction of Cl2•- radical anions with ethanol. Surface kinetic information is obtained by monitoring absorption of the transient species, i.e.,Cl2•-, following laser flash photolysis of a K2S2O8/ NaCl/ethanol/H2O solution. The goal of this research is aimed to better understand and evaluate the role of air-water interface processes of atmospheric interest. Experimental Section Methods. The experimental methodology couples radical production by laser flash photolysis (LFP) with time-resolved detection of products using UV diffuse reflectance (DR). A highpressure xenon arc lamp was used in time-resolved optical absorption by a transient species. The lamp’s output wavelength can be filtered to match a specific absorption region of the species of interest. Therefore, the UV DR-LFP technique may be used to monitor concentrations of specific species in solution. First employed in the 1980s to probe solid heterogeneous systems, the diffuse reflectance-laser flash photolysis technique has become a sensitive tool for understanding the kinetics and mechanisms of photoreactions on many different kinds of solid surfaces and interfaces.19-24 The diffusion reflectance approach used in this study is similar to one used in previous studies of heterogeneous reactions traditionally performed on solid surfaces (see, for example, refs 5, 19, and 25-27). However, to date, there is no work reported that uses UV diffuse reflectance spectroscopy to directly study reaction kinetics of atmospheric interest at the air-water surface. To our knowledge, this is the first study to use the UV diffuse reflectance time-resolved technique to directly “probe” reaction kinetics on a transparent medium such as a water surface. The UV DR-LFP apparatus used for the Cl2•- + ethanol experiments is shown schematically in Figure 1. The experiments involve time-resolved detection of Cl2•- radical anion by optical absorption spectroscopy at λ ≈ 350 nm following 248 nm laser flash photolysis of K2S2O8/ NaCl/ethanol/H2O solution according to
S2O82- + hν (λ ) 248 nm) f 2SO4•-
(1)
SO4•- + Cl- f Cl• + SO42-
(2)
Cl• + Cl- / Cl2•-
(3)
Cl2•- + C2H5OH f products
(4)
Cl2•- + Cl2•- f products
(5)
Cl2•- f loss
(6)
Cl• f loss
(7)
Reaction 2 has been shown to be the rate-determining step in the production of the Cl2•- radical anion with a reported rate coefficient of (3.3 ( 0.5) × 108 M-1 s-1.28 The reaction of the Cl• radical with ethanol has been omitted from the reaction mechanism above. However, this reaction would not alter the outcome of this study. In reaction mechanism 1-3 the SO4•radical anion is converted to Cl2•- in the presence of Cl- with a yield of >99.5% in less than 0.2 µs.29 The decay of Cl2•- is mapped out on a storage oscilloscope (Tektronix, type 2430A) following a synchronized (under Stanford Research Systems DG535 control) delay between the 248 nm photolysis laser flash and the Xe flash lamp probe pulse. The pulse widths of the 248 nm and Xe flash lamp were 20 ns and 1-11 ms, respectively, while the time scale for the occurrence of the reaction of Cl2•- with ethanol was typically 0.1-1 ms. All experiments were carried out under strictly pseudo-first-order conditions with Cl- and ethanol concentrations in excess over the Cl2•- concentration. Details of the experimental procedure that was employed to study the reaction of Cl2•- with ethanol are given below. A cylindrical Teflon reaction cell with an internal volume of ∼200 cm3 was used in all of the Cl2•- + ethanol experiments.
Reaction of Cl2•- Radical Anions with Ethanol The internal walls of the reaction vessel were blackened to minimize scattering of radiation. The solution droplet was supported on the flat top of a 6 mm o.d. Teflon tube in the center of the reaction cell. This geometry resulted in a nonspherical droplet of about 8 mm in diameter. Two quartz lenses (focal length 5 cm) and six quartz windows, 2.54 cm o.d., evenly spaced were fixed to the main body of the cell. The cell was maintained at room temperature. A copperconstantan thermocouple with a stainless steel jacket was inserted into the solution reservoir and the reaction cell through a seal, allowing measurement of the solution and the droplet’s surrounding gas medium temperature under the precise experimental conditions employed. The geometry of the reaction vessel was such that it allowed for the photolysis laser and the probe laser beams to enter ∼30° to one another. Two separate photomultiplier tubes were used to monitor the reflected and aqueous-phase, i.e., bulk liquid, signals. One photomultiplier (Hamamatsu H7732-10) was placed at ∼30° relative to the probe beam. The second photomultiplier (Hamamatsu H7826) was positioned on the same axis as the Xe lamp beam. Such detector geometry allowed for the “reflected” and the “bulk” (i.e., transmitted) signals to be collected under the same experimental conditions. Radiation from a Lambda Physik EMG 101 KrF excimer laser (λ ) 248.5 nm) served as the photolytic light source for the study of the reaction of Cl2•- radical anions with ethanol. Fluences of 248.5 nm laser radiation utilized in this study were typically 100-130 mJ cm-2, and the laser pulse width was 20 ns. The xenon lamp’s output wavelength was filtered to obtain a maximum output radiation at λ ≈ 350 nm to match the maximum absorption region of the Cl2•- radical anion.30 The photolysis laser was triggered at a specific delay time after the probe beam pulse so that the photolysis laser fired in the “plateau” of the Xe flash lamp. Reflected absorption of the Cl2•- radical anion at λ ≈ 350 nm was collected by a quartz lens on the axis ∼30° to the probe beam, passed through two 350 nm interference filters and a 20% acetonitrile/methanol filter, and imaged onto the photocathode of a photomultiplier tube. The bulk liquid signal was collected on the same axis as the probe beam passing through the droplet using the geometry of lenses and filters as described above. Given the detector geometry listed above, special care had to be taken so that the reflected signal did not include signal from the bulk-phase processes. That is, the reflected signal may contain information from the bulk-phase processes which result from the internal and diffuse reflections of the analyzing light within the water droplet. As a result, extensive experiments were carried out to ensure that the bulk-phase contribution to the reflected signal was negligible. For example, in one series of preliminary experiments the PMT that measured the reflected signal was placed at various positions around the water droplet. While the observed reflected signal intensity changed, the observed kinetics was always the same. If internal reflections and subsequent transmission of light out of the droplet were significant, then the observed kinetics would have been different at different positions around the water droplet. In another series of preliminary experiments, the excimer laser power and the cross-sectional area of the analyzing light were varied. Similarly, the observed reflected signal intensity varied, but the kinetics remained the same. The maximum intensity of the reflected signal was found to be at an angle of ∼30°. This angle of reflection is consistent with the theoretical work of the reflection-absorption calculations performed by Dluhy.31 Dluhy’s theoretical calculations of the reflection absorption for a
J. Phys. Chem. A, Vol. 107, No. 14, 2003 2499 monolayer on water suggest that the optical angle for experimental determination of the reflection spectra of thin films on water is in the range 0-40°.31 Under the experimental conditions employed, assuming a conservative value of 2% reflection32,33 for the water droplet surface, and using some simple algebra and trigonometry, we can calculate that the reflected signal intensity was at least 106 greater than the signal intensity contributed from the bulk-phase processes. However, the reflected signal may still contain information from the bulkphase processes which result from the fact that the diffuse reflectance-laser flash photolysis technique is not a purely surface specific method. That is, the penetration depth (depth of the solution surface analyzed by the experiment)32,33 of the analyzing light may be tens of nanometers deep. If the sounding depth is a few tens of nanometers, then there should be a contribution from the bulk phase just below the surface. This point will be discussed later. Before the photolysis laser was discharged, the “background” steady light level of the Xe lamp was measured by a sampleand-hold circuit using a multimeter. This procedure allowed for the absorption baseline to be obtained. The output pulse from the PMT was passed through a high-speed current amplifier/ discriminator (FEMTO HCQ-200M-20K-C), fed to a differentiating circuit that “backed-off” the steady light signal, and then recorded on a storage oscilloscope. The Tektronix storage oscilloscope had a signal-averaging capability and a maximum sampling rate of 100 MHz. The stored signal was digitized and transferred from the oscilloscope to the microcomputer using an IEEE-488 connection. The full absorption signal was then reconstructed from the steady and transient signals. Up to 16 single-shot experiments were averaged to map out a single Cl2•radical anion temporal profile over two 1/e times. All experiments were carried out under “static” conditions. However, the solution was allowed to flow using a Watson Marlow 313S liquid pump, and the droplet was replenished after each photolysis laser/flash lamp shot. Therefore, each surface area and volume element of the reaction solution were subjected to only one laser/flash lamp shot, thus preventing the buildup of reaction products on the reaction surface and in the bulk liquid. Concentrations of each component in the reaction mixture were determined from the appropriate mass and volume measurements. The reaction solution was allowed to flow into the reaction cell from its blackened and foil-coated 1 L storage container. The concentration of SO4•- in the aqueous phase was not directly measured but was calculated on the basis of experimentally measured and certain known parameters, namely, the quantum yield for SO4•- radical production from 248 nm photolysis of S2O82-, absorption cross section of the S2O82anion at 248 nm, and laser photon fluence at 248 nm. Reagents. The reagents used in this work had the following stated minimum purities: K2S2O8 (Aldrich, ACS reagent, >99%); NaCl (Aldrich, ACS reagent, >99%; ethanol (Prolabo, absolute, >99.8%). All solutions were prepared using water with a resistivity of >18 MΩ cm. Deionized water was prepared by passing tap water through a reverse osmosis demineralization filter (ATS Groupe Osmose) followed by a commercial deionizer (Millipore, Milli-Q50). All solutions were used within 1 h of their preparation. All experiments were performed with a pH value of ∼5.8. A possible impurity in K2S2O8 was sulfuric acid. However, if potassium persulfate was predominantly responsible for controlling the pH of our solutions, then a pH of 5.8 would be equivalent to a sulfuric acid content of ∼0.003%.
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Results and Discussions Adding ethanol to water modifies the surface tension of the surface. As a result, the concentration profile near the surface is altered. The concentration of ethanol at the water droplet surface, [ethanol]σ, was estimated on the basis of the work of Donaldson and co-workers12,34-36 and Strey and co-workers37 using a Langmuir-type isotherm:
[ethanol]σ ≡ Γ )
Γsat[ethanol]bulk b + [ethanol]bulk
(I)
In eq I, Γ and Γsat are the surface coverage and saturated surface coverage, respectively, and the parameter b is related to the rate coefficients for adsorption and desorption from the surface into the liquid bulk. It should be noted that eq I adopted from the work of Donaldson and co-workers12,34-36 and Strey and coworkers37 is an approximation. The left side of eq I, Γ, is not really the surface coverage (concentration) of ethanol but relative adsorption of ethanol with respect to water. However, for low concentrations of bulk ethanol, and assuming monolayer segregation, the surface excess and surface concentrations are approximately the same. The concentration of ethanol at the interface, [ethanol]σ, was calculated using Gibbs approach38 where the relative surface excess of species i was expressed as
ΓH2O,i )
( ) ∂σ ∂µi
T,µj*1
( )( )
) -
ai ∂σ RT ∂ai
Figure 2. Typical (a) bulk and (b) surface Cl2•- absorbance temporal profiles observed in the study of the reaction of Cl2•- with ethanol. The photolysis laser was fired at time 0. Experimental conditions: T ) 298 K; [NaCl] ) 50 mM; [K2S2O8] ) 5 mM; [ethanol] ) 0.3 M. The number of laser shots averaged was 16.
(II)
for adsorption from the bulk liquid.34 In eq II ΓH2O,i is the surface coverage of species i, σ is the surface tension, µi is the chemical potential of species i, ai is the activity of species i, R is the gas constant, and T is the absolute temperature. (Hereinafter, ΓH2O,i will be abbreviated to Γ.) Here, similar to the work of Donaldson and Anderson on adsorption of C1-C4 alcohols, acids, and acetone gases at the air-water interface, solution concentrations were used rather than activities.34 The use of solution concentrations is consistent with the work of Strey and co-workers on the necessity of using activities in the Gibbs equation.37 Equilibrium surface tensions of aqueous ethanol solutions were taken from the work of Strey and co-workers.37 On the basis of the work of Donaldson and Anderson, the following exponential polynomial function was used to fit surface tension data:34
σ ) σ0e-a1[ethanol] + a2[ethanol] + a3[ethanol]2 + a4[ethanol]3 (III) In eq III, [ethanol] is the aqueous-phase ethanol concentration (mol L-1) and σ0 is the surface tension of pure water (σ0 ) 72.0 dyn cm-1). The derivative of eq III was then used to calculate the relative surface excess via the Gibbs equation:34
d (σ) ) -σ0a1e-a1[ethanol] + a2 + 2a3[ethanol] + d[ethanol] 3a4[ethanol]2 (IV) The saturated surface coverage, Γsat, was then obtained from least-squares analysis to a Langmuir isotherm. The Γsat and b are found to be 3 × 1013 molecules cm-2 and 0.3, respectively. Once Γsat, the parameter b, and the bulk ethanol concentration were known, the “real” surface ethanol concentration, Γ, was calculated.
Figure 3. Typical (a) bulk and (b) surface Cl2•- first-order decays observed in the study of the Cl2•- + ethanol reaction. The photolysis laser was fired at time 0. Experimental conditions: T ) 298 K; [NaCl] ) 50 mM; [K2S2O8] ) 5 mM; [ethanol] ) 0.3 M. The number of laser shots averaged was 16. Fits are obtained from linear least-squares analyses and give the following first-order decays (103 s-1): (a) 87, (b) 120.
All experiments were carried out under pseudo-first-order conditions with Cl- and ethanol concentrations in excess over the Cl2•- radical anion concentration. Typical liquid bulk Cland S2O82- concentrations were 50 and 5 mM, respectively, while aqueous-phase ethanol concentrations were in the range from 0.11 to 0.64 M. Some typical surface and bulk liquid Cl2•radical anion temporal profiles observed following laser flash photolysis of K2S2O8/NaCl/ethanol/water mixtures are shown in Figure 2. Typical first-order plots for the “surface” and bulk channels are shown in Figures 3 and 4. The bimolecular plots for the reaction of Cl2•- with ethanol are shown in Figure 5. The effective surface and bulk liquid rate coefficients, kobsd(surface) and kobsd(bulk), respectively, are obtained from the variation of the corresponding pseudo-first-order rate coefficient, k′ (for the bulk) and k′σ (for the surface), with [ethanol] at constant concentrations of Cl- and S2O82-. The plot of the pseudo-first-order rate coefficient k′σ versus the ethanol surface concentration from which the rate coefficient for the surface reaction of the Cl2•- anion with ethanol was extracted is shown in Figure 6. The observed surface rate coefficient for the reaction of the Cl2•- radical anion with ethanol is found to be strictly proportional to the ethanol surface concentration (see Figure 6). Fits to plots of kobsd(surface) and kobsd(bulk) vs [ethanol] in the aqueous phase are described by the following expressions:
Reaction of Cl2•- Radical Anions with Ethanol
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Figure 4. Typical Cl2•- first-order temporal profiles observed for the (A-C) bulk and a-c surface in the study of the reaction of Cl2•- with ethanol. The photolysis laser was fired at time 0. Experimental conditions: T ) 298 K; [NaCl] ) 50 mM; [K2S2O8] ) 5 mM. [Ethanol] (M): (A, a) 0.08, (B, b) 0.15, (C, c) 0.3. The number of laser shots averaged was 16. Solid lines are obtained from least-squares analyses that give the following best-fit k′ parameters (103 s-1): (A) 55, (B) 74, (C) 110, (a) 77, (b) 104, (c) 160.
Figure 5. k′ versus [ethanol] for (a) bulk and (b) surface data obtained in the studies of the reaction of Cl2•- with ethanol. Fits are obtained from a linear least-squares analysis and Langmuir equation and give the following bimolecular rate coefficients (105 M-1 s-1): (a) 1.77 ( 0.34 and (b) 4.45 ( 0.80, respectively. Uncertainties are 2σ and represent precision only.
kobsd(surface) ) k′σ[ethanol]σ + k0σ
(V)
kobsd(bulk) ) k′[ethanol] + k0
(VI)
In eqs V and VI, k′σ and k′ are the second-order rate coefficients for the decay of Cl2•- at the surface and in the aqueous phase, respectively, [ethanol]σ and [ethanol] are ethanol concentrations at the surface and in the bulk liquid, respectively, and k0σ and k0 are surface and bulk, respectively, pseudo-first-order rate coefficients for the Cl2•- decay in the absence of ethanol. It should be noted in Figure 5 that the kinetics at the surface do not increase linearly with the bulk liquid ethanol concentration. It is observed that the nonlinear behavior of the experimental kinetic data for the surface shows a Langmuir-type dependence on concentration. Therefore, the data presented in Figures 5 and 6 indicate that the reaction Cl2•- + ethanol at the air-water surface can be described by a dynamic Langmuir model. Accordingly, since the kinetics at the interface are consistent with a Langmuir-type description, we report with confidence that we are, indeed, observing the diffuse reflectance signal originating at the surface (or just beneath it) and not in the bulk liquid. Also, it should be noted that the deviation from the first-
Figure 6. Pseudo-first-order rate coefficient k′σ versus ethanol surface concentration from which the rate coefficient for the surface reaction of the Cl2•- anion with ethanol was extracted. The fit is obtained from a linear least-squares analysis and results in a bimolecular rate coefficient equal to (9.22 ( 0.82) × 10-9 cm2 s-1 molecule-1 for an arbitrary penetration depth of 0.8 nm. Uncertainties are 2σ and represent precision only.
order behavior of the decay of the Cl2•- radical anion may arise as a result of the fast self-recombination of Cl2•- anions (i.e., reaction 3). The rate coefficients for reaction 3 have been reported to be (1.8 ( 0.1) × 109 M-1 s-1 29 at I f 0 (I ) ionic strength) and T ) 298 K and 7 × 108 M-1 s-1.39 Under the experimental conditions employed in this work we estimate a “first-order” contribution from reaction 3 to be
